Image evaluation methods of this type and the corresponding items are known.
Thus, for example, an image evaluation method of this type is known from the technical article “Quantitative Analyse von koronarangiographic Bildfolgen zur Bestimmung der Myokardperfusion” [Quantitative analysis of coronary angiographic image sequences for determining myocardial perfusion] by Urban Malsch et al., which appeared in “Bildverarbeitung für die Medizin 2003—Algorithmen—Systeme—Anwendungen” [Image processing for medicine 2003—algorithms—systems—applications], Springer Verlag, pages 81 to 85. In the case of this image evaluation method, a computer determines with the aid of the projection images a two-dimensional evaluation image which comprises a plurality of pixels, and outputs the evaluation image via a display device to a user. The pixels of the evaluation image correspond to those of the projection images. The computer undertakes, with the aid of the temporal course of the pixel values of the projection images, to assign a pixel value to the pixels of the evaluation image, the pixel value being characteristic of the time of maximum contrast change.
The doctrine of the above-mentioned technical article is described in the context of angiographic examinations of the coronary arteries of the human heart. This type of examination is one of the most important diagnostic tools in cardiology today. Additional information such as the determination of flow velocity or myocardial perfusion is further information which can in principle be obtained by means of angiography. The essential diagnostic evidence here is the perfusion of the myocardium.
Today, a number of other non-invasive methods of examination such as PET, SPECT, MR or contrast-medium-aided ultrasound have also become established. These methods of examination offer the facility for quantifying, in addition to other parameters, the perfusion status of the myocardium. These methods are generally applied in stable angina pectoris cases or in order to assess the risk after a myocardial infarction.
Arteriosclerosis, the trigger of myocardial infarctions, is not only a chronic disease which progresses slowly but also an extremely dynamic one. Here, the vascular wall not only becomes more rigid and thicker as a result of fat deposits, proliferations of smooth muscle cells and connective tissue-like rebuilding with calcification. Rather, active inflammatory processes with dissolving of tissue by matrix proteinases, apoptotic cell death and neovascularization determine the instability of the lining (plaque). A plaque rupture in a coronary artery, together with exposure of highly active arteriosclerotic material to flowing blood, triggers in extreme cases an acute coronary occlusion and, as a consequence, a myocardial infarction.
In the last few years, there have been increasing indications and evidence that a coronary plaque rupture does not always lead to a vascular occlusion and thus to a myocardial infarction, but may also lead to an embolization of arteriosclerotic material into the coronary microcirculation. Such a coronary microembolization can occur spontaneously as well as in the context of coronary interventions. In particular, a coronary intervention can also trigger a microembolization.
For an assessment of the therapeutic outcome of an intervention, it would therefore be advantageous to be able to monitor the improvement in perfusion and/or the occurrence of microembolization and microinfarctions during the actual intervention. It would consequently be advantageous if quantification of the perfusion were added to the other diagnostic parameters in the catheter laboratory, as this would make it possible to obtain all relevant information in one examination and thus to achieve an improvement in the monitoring of treatment.
Quantification of the perfusion of the myocardium by means of angiographic methods is, however, problematical, since the angiographically observable cardiac vessels have a diameter of almost a millimeter or more. These observable vessels terminate, however, in millions of tiny capillary vessels which have diameters of only a few micrometers. However, the flow dynamics and distribution in the capillary vessels are ultimately determined by the blood supply of the cardiac muscle. Drawing conclusions from macroscopic perfusion as to the dynamics of perfusion in the capillary vessels is therefore, strictly speaking, inadmissible, even though it is often done.
In order to record perfusion of the myocardium, various methods are known, in particular contrast echocardiography, magnetic resonance tomographic diagnostics and SPECT.
The echocardiographic determination of global and regional function is a firm component of non-invasive cardial functional diagnosis. Dynamic and pharmacological stress echo cardiography are used particularly in cases of ischemia and in vitality diagnostics and contribute to the indication of revascularizing measures in cases of chronic coronary heart diseases. Contrast-specific imaging methods have in this context recently enabled amplification of the signal from the intramyocardial blood pool, on the basis of which statements can be made with regard to myocardial perfusion. Current real-time methods even enable the simultaneous assessment of wall motion and myocardial perfusion with high spatial resolution.
Magnetic resonance tomographic diagnostic methods for coronary heart diseases are based on the evidence of pharmacologically induced perfusion or wall-motion disorders. Contrast-medium-aided first-pass perfusion measurement at rest and under pharmacological stress is the preferred procedure today for assessing myocardial perfusion. Here, drugs are used which lead to dilation of the unaffected coronary arteries and consequently, due to the raised blood flow in these dilated coronary arteries, result in an increase of the lower perfusion rate in the area supplied by a stenosis-affected coronary artery.
SPECT is a nuclear medical method. Tc-99m is nowadays used for this purpose as a contrast medium alongside thallium-201 chloride. Myocardial perfusion scintigraphy records the perfusion of the cardiac muscle under ergometric and pharmacological stress and at rest. In the process, reversible ischemias can be differentiated from permanent perfusion disorders or myocardial scars. A prerequisite for this method is an optimized tomographic examination technique.
Acute myocardial infarction represents a cardiological emergency situation in which a rapid diagnosis and treatment are required. In this emergency situation, an examination of the patient using magnetic resonance tomographic methods, SPECT methods or contrast echo cardiography is not as a general rule possible. Further problems emerge if for different reasons it has not been possible to carry out a perfusion measurement in advance. In all these cases angiographically based cardiac perfusion imaging would provide an important tool.
In angiographically based cardiac perfusion imaging, long recordings are made, the recordings lasting until such time as the contrast medium has flowed through the coronary arteries and is visible in the myocardium itself. This latter phase is referred to as the “myocardial blush”. Assessment of the “myocardial blush” serves in providing evidence of the vascular supply to the heart and for example in rating the success of treatments and/or a risk profile for the patient.
In order to make the blood flow dynamics in large vessels and in the capillary vessels measurable and thereby comparable, various gradation systems are known which divide up the continuum of conditions into discrete classes. Some of these classifications describe the macroscopic circulation of blood, others the circulation of blood in the capillaries. The most-used classifications were drawn up by the scientific organization “Thrombolysis in Myocardial Infarction” (TIMI). These classifications are deemed to be the standard. In multi-center studies in which reproducible and comparable results are what particularly matters, the TIMI classifications are frequently used. The classifications are, however, complex and can be applied only in a time-consuming manner. They are therefore not generally used in routine clinical work.
By far the most frequently used method in the prior art is the visual assessment of “myocardial blush” on a screen. This procedure is often used for multi-center studies. A prerequisite for this procedure is that the angiographic recording is long enough in order to be able to see the entry and washout of the contrast medium. However, the visual assessment requires a lot of experience and is in practice carried out only by TIMI-blush experts, as they are known.
There are also various procedures known in which an attempt is made to carry out this subjective and personal visual assessment with the aid of computers. An example is to be found in the aforementioned technical article by Urban Malsch et al.
The procedure in the aforementioned technical article represents a good initial attempt but still displays shortcomings. For example, it is particularly necessary to identify the vessels of the vascular system in the projection images in order to mask out these vessels when analyzing the “myocardial blush”. It is also necessary in the case of the procedure in the technical article to work with DSA images. This gives rise to a significant risk of artifacts, to avoid which compute-intensive methods are in turn required in order to compensate for motion.